SV40-transformed cells have been shown to exhibit increased phosphorylation of endogenous proteins when assayed by autoradiography. We now find a good correlation between the expression of increased phosphorylation in transformed cells and a significantly increased sensitivity of these cells to levels of exogenous zinc which are not toxic to nontransformed cells which do not show increased phosphorylation of their cellular proteins. Zinc is known to be an inhibitor of cellular protein kinases which may be responsible for the increased protein phosphorylation observed in the transformed cells.